Extraction of lateral eigenmode properties in thin film bulk acoustic wave resonator from interferometric measurements

A heterodyne laser interferometer is used to study acoustic wave fields excited in a 1.8 GHz AlN thin film bulk acoustic waveresonator. The electrical response of the resonator exhibits a strong thickness resonance onto which spurious modes, caused by lateral standing plate waves, are superposed. Optical interferometermeasurements are used to extract dispersion curves of the laterally propagating waves responsible for the spurious responses. A discrete eigenmode spectrum due to the finite lateral dimensions of the resonator is observed. An equivalent circuit model for a multimode resonator is fitted to the mechanical resonator response extracted along a single curve in the dispersion diagram, and is used to determine properties, such as Q-values, of the individual lateral eigenmodes.Measuredwave field images, extracted dispersion curves, and the eigenmode spectrum with the model fitting results are presented.Peer reviewe

Characterization of energy trapping in a bulk acoustic wave resonator

Acoustic wave fields both within the active electrode area of a solidly mounted 1.8 GHz bulk acoustic waveresonator, and around it in the surrounding region, are measured using a heterodyne laser interferometer. Plate-wave dispersion diagrams for both regions are extracted from the measurement data. The experimental dispersion data reveal the cutoff frequencies of the acoustic vibration modes in the region surrounding the resonator, and, therefore, the energy trapping range of the resonator can readily be determined. The measureddispersionproperties of the surrounding region, together with the abruptly diminishing amplitude of the dispersion curves in the resonator, signal the onset of acoustic leakage from the resonator. This information is important for verifying and further developing the simulation tools used for the design of the resonators. Experimental wave field images, dispersion diagrams for both regions, and the threshold for energy leakage are discussed.Peer reviewe